Recent developments in cell/tissue engineering have recognized benefits to growing and studying cells in dynamic environments. Spinner flasks, rotary devices, perfusion bioreactors, or fluid sheer chambers have all been used to enhance nutrient and metabolite diffusion to and from cells. The mechanical aspects of fluid sheer forces have also been shown to trigger second messenger signals and alter cellular gene expression. While these new culture conditions have been recognized to affect cell functions (growth, signaling, morphology, differentiation, etc.), devices for studying these environments have not been translated to high throughput platforms. Furthermore, systems that incorporate three-dimensional scaffolds with highly aligned pores for long-range control over fluid flow paths have also not been established.
Fluid flow was first established as a regulator of cellular gene expression in two-dimensional culture systems with flowing culture medium over cells adherent to glass slides. Cells respond to the fluid sheer by aligning in the direction of the force, and altering their gene expression. These two-dimensional devices are now commercially available from, for example, Flex-Cell International, as well as other vendors. Fluid flow studies have recently been translated to three-dimensional scaffolds, and it has been established that fluid sheer is another important factor in maintaining hepatocyte and bone differentiation. The true importance of fluid flow as an environmental signaling factor, however, has not been fully appreciated because it is difficult to screen against or study in conjunction with a plethora of other environmental cues that are known to alter cell function including but not limited to signaling factors such as growth factors, ECMs, cytokines, media factors, and small molecules to name a few. For example, to date, all of the devices designed to study how these forces affect cell cultures are one-pot or single chamber devices. These devices may be utilized to study how rotation or fluid sheer forces affect cells under one condition at a time, but not under different or varying conditions, which greatly limits the utility of these devices. Accordingly, current devices are not suitable for performing medium or high throughput experimentation for optimization of conditions for controllable cell phenotype, or for testing substances such as molecules of unknown function for altering specific functions in highly relevant cell or engineered tissue cultures.
Furthermore, in the field of drug discovery, the use of primary human cells to study ADMETox (ADMETox is an acronym for set of analyses that measure the absorption, distribution, metabolism, elimination and toxicity of a drug candidate) properties of drugs is highly desirable. This is due to the fact that whole animal studies are expensive, and results are not always predictive of responses in man. In vitro study of primary human cells is attractive due to the economics of the approach, and the fact that data from human cells should be more relevant than animal data. Unfortunately, the culture of primary human cells is extremely difficult for most cell types, and there are few model systems that are capable of creating relevant models of in vivo tissues and organs. As an intermediate between whole animals and primary cells, tissue or organ slices offer an alternative that keeps cells in their native setting (not dissociating them from their microenvironment), while allowing for in vitro testing of xenobiotic effects on cell viability, metabolism, and other ADMET-type aspects that one desires. For example, liver slices are often utilized for measuring liver-specific drug toxicity, as well as CYP induction.
In vitro culture of tissue slices also has several challenges. For example, one significant challenge is the high metabolic rates and nutrient requirements that tissue slices need in vitro. Since the tissue slices require a large nutrient load, it is necessary to culture these slices in large quantities of medium. However, the more medium that one adds to a culture increases the diffusion distance of oxygen to the extent that the rate of consumption by the tissue is greater than the diffusion of oxygen, leading to hypoxic conditions and cell death. There is, therefore, a great need for bioreactor-type devices that enhance nutrient and metabolite transport while maintaining a medium-to-high throughput parallel testing format.
A need exists for a system and method for culturing cells under fluid perfusion in medium to high throughput format, to test and/or discover how new environments alter the ability of cells to respond to other chemical or physical cues in the presence of fluid sheer and to facilitate the systematic and high throughput discovery of dynamic cell culture conditions for cell growth and differentiation, and then utilizing these optimized environments for creating in vitro engineered tissues for therapeutic, diagnostic, or research purposes. A need also exists for a perfusion system that allows for the dynamic and multiplexed culture of a variety of tissue or organ slices for ADMET and tissue culture applications.